Organic LEDs (also known as OLEDs) are optoelectronic components that are the subject of rapid development and that are used essentially in making flat screens.
Whereas a “conventional” LED is made of inorganic semiconductor materials, an OLED is made of layers of organic materials. This leads to fabrication technology that is much simpler and less expensive to implement. In particular, the organic materials constituting OLEDs may be deposited easily on large areas in order to make flat screens at low cost.
FIG. 1 is a diagram for explaining the operation of a very simple OLED constituted by a stack of four layers:                a transparent anode A, e.g. made of indium tin oxide (ITO) deposited on a substrate S made of glass;        a hole transport layer HTL, e.g. made of Spiro TTB doped with F4TCNQ, or with MoO3;        an emitting layer EL, e.g. made of AlQ3 or of TMM004 doped with Irppy;        an electron transport layer ETL, e.g. made of Bphen doped with Ca or with CsCo3; and        a reflective cathode, e.g. made of Ag or Al.        
The layer HTL conducts holes “h” injected into the structure by the anode; it performs the function of the p-doped layer in conventional LEDs. Conversely, the layer ETL conducts electrons “e” injected into the structure by the cathode; it thus performs the function of the n-doped layer in conventional LEDs. When the electrons and the holes meet within the emitting layer, they form excitons EX, i.e. pairs linked by Coulomb interaction, which may recombine by a radiative process, thereby emitting a photon. The emitted photons leave via the anode and the transparent substrate, possibly after being reflected by the metal cathode (“downward emission”). In a variant, the cathode may be transparent, being constituted by a very fine metal layer, and the anode may be reflective, thus providing a diode having “upward” emission. Blocking layers (not shown) may be provided in order to limit leakage of carriers beyond the light-emitting layer: excitons that are generated close to the electrodes usually recombine in non-radiative manner, thereby reducing the efficiency of the device. This phenomenon is known by the term “quenching”.
One of the main drawbacks of OLEDs is the broad spectral width of the radiation they emit, thus making it impossible to obtain colors that are sufficiently saturated.
One solution to that problem consists in using hybrid diodes, including quantum dots that are associated with the organic semiconductor layers.
A quantum dot is a nanoparticle of inorganic semiconductor material of a size that is sufficiently small to enable excitons to be confined in three dimensional space. Typically, a quantum dot is constituted by a core surrounded by a shell of semiconductor material that has a forbidden band that is broader than that of the core. Molecules may be deposited on the shell so as to modulate the chemical and physicochemical properties of the quantum dot, e.g. its ability to remain in suspension in a solvent. Quantum dots are light-emitting and they present an emission band that is relatively narrow compared with organic emitters: when appropriately incorporated as a light-emitting element in an optoelectronic component as a light-emitting element, they therefore enable highly saturated colors to be obtained.
FIG. 2 shows a simplified structure for an organic LED including, between its HTL and ETL, a monolayer BQ of quantum dots. In this device, the HTL and ETL inject holes and electrons respectively into the quantum dots, where those carriers become linked to form excitons. A fraction of the excitons recombine by a radiative process, emitting light of a spectrum that depends exclusively on the properties of the quantum dots, and not on the properties of the organic layers HTL and ETL.
In known manner, it is necessary for the energy levels of the layers HTL and ETL to be adapted to the energy bands of the quantum dots in order to enable carriers to be injected efficiently.
Hybrid organic and quantum dot LEDs and methods of fabricating them are described in detail in the following articles:                Polina O. Anikeeva, Jonathan E. Halpert, Moungi G. Bawendi, Vladimir Bulovic, “Quantum dot light-emitting devices with electroluminescence tunable over the entire visible spectrum”, Nano Letters 2009, Vol. 9, No. 7, pp. 2532-2536; and        Seth Coe-Sullivan, Jonathan S. Steckel, LeeAnn Kim, Moungi G. Bawendi, Vladimir Bulovic “Method for fabrication of saturated RGB quantum dot light-emitting devices” Proc. SPIE 5739 (2005), pp. 108-115.        
The efficiency of such devices is limited by the spin statistics of the carriers. It is well known that excitons come in two forms: singlet states in which the total spin angular momentum is zero (S=0) and triplet states in which the total spin angular momentum is one (S=1). There are three triplet levels for one singlet level; in other words 75% of the excitons are in a triplet state and 25% in a singlet state.
The fundamental state reached after recombination presents a total spin angular momentum that is equal to zero; consequently, symmetry configurations prevent triplet states recombining by an electric dipole transition. As a result, these states present a lifetime that is much longer than that of singlets, and they usually recombine by a non-radiative process. Because of that, in fluorescent materials, only singlet excitons contribute to light emission, so efficiency cannot exceed 25%.
Things are different in phosphorescent materials, where strong spin-orbit coupling induces triplet and singlet states to mix and enables the triplet state to de-excite in radiative manner by inter-system conversion. This effect is sometimes made use of in OLEDs having a phosphorescent emitting layer. Another technique consists in mixing fluorescent molecules with a phosphorescent “sensitizer” in a non-light-emitting organic matrix. Under certain conditions, the triplets of the phosphorescent sensitizer may yield their energy by non-radiative transfer of the Förster type to the fluorescent molecules, that provide the emission of the device. That technique is described in the article by M. A. Baldo, M. E. Thompson, and S. R. Forrest “High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer”, Nature 403 (2000), pp. 750-753.
Unfortunately, most quantum dots do not present an efficient inter-system conversion path, thereby limiting the yield of hybrid organic and quantum dot diodes. It is not easy to adapt the technique proposed in the above-mentioned article by M. A. Baldo et al. to such devices.
An article by Polina O. Anikeeva, Jonathan E. Halpert, Moungi G. Bawendi, and Valdimir Bulovic “Photoluminescence of CdSe/ZnS core/shell quantum dots enhanced by energy transfer from a phosphorescent donor”, Chemical Physics Letters 424 (2006), pp. 120-125 mentions exciton transfer from a phosphorescent material to quantum dots. That technique is advantageous only for quantum dots that provide efficient inter-system conversion, which is not true in general. In addition, the device described in that article is photoluminescent and not electroluminescent, and it makes use of a thick phosphorescent layer. The use of a layer that is that thick for making an LED would lead to components that consume a large amount of energy.
Another phenomenon limits the yield of hybrid organic and quantum dot LEDs. The efficiency of such devices depends on the capacity of the carriers to occupy the levels of the quantum dots. Unfortunately, there is considerable misalignment between the valance and conduction bands quantum dots and the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of adjacent organic semiconductors. Because of that misalignment, only a small fraction of the pairs that form actually recombine in the quantum dots. That phenomenon is accentuated by the very small thickness of the quantum dot layer BQ, which allows the carriers to escape. The prior art provides no solution to that problem.